1. Field of the Invention
The present invention relates to an AGC circuit mounted in a television tuner, and more particularly, to an AGC circuit which is hard to be influenced by the level of an input signal.
2. Description of the Related Art
A conventional automatic gain control (AGC) circuit, for example, includes a voltage dividing circuit which allows a change in an impedance changed by a AGC voltage to be opposite to a change in the input impedance of an amplification transistor, a current negative feedback circuit which allows the change in the impedance changed by the AGC voltage to be equal to the change in the input impedance of the amplification transistor, a variable attenuator composed of a pin attenuator, and an AGC voltage generating circuit, and suppresses a change in the input impedance of an amplifier to prevent reduction of input return loss and deterioration of noise figure (NF) (for example, see Patent Document 1).
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2001-244766 (FIG. 1)
However, in the conventional circuit, it is difficult to obtain necessary gain reduction (GR) by a frequency.
Accordingly, in the conventional circuit, the variable attenuator composed of the pin attenuator and an operational amplifier are required. Therefore, when the AGC circuit having the above-described configuration is embodied on an integrated circuit, electronic parts other than the amplification transistor and the negative feedback circuit are required and thus the configuration becomes complicated.
In a case where the negative feedback is simply applied to reduce the gain of the amplification circuit and to ensure the GR without using the variable attenuator or the operational amplifier, the NF deteriorates when the level of an input signal is small and the waveform of the signal is distorted when the level of the input signal is large.